This invention relates to an improved process for preparing polycrystalline metal oxide varistors. More particularly, the invention relates to a process for preparing polycrystalline metal oxide varistors having one or more dimples which reduces the thickness of the varistor in the dimples whereby the configuration permits the breakdown voltage of the varistor to occur at a lower voltage. The term "breakdown" is not meant to denote device failure, but is used to designate a value of voltage across the device beyond which the current through the device increases greatly. That is, for voltage values below the breakdown voltage, the device behaves like an ohmic resistor of very large value (in the megohm range) but when the breakdown voltage is exceeded, the device behavior is very much like that of a low resistance conductor. These devices exhibit a very nonlinear current voltage characteristic.
Metal oxide varistors are sintered ceramics composed principally of zinc oxide with a mixture of various other metal oxides added. These other oxides are typically bismuth trioxide, cobalt trioxide, manganese dioxide, antimony trioxide, and tin dioxide, each being present to the extent of approximately 1/2 to 1 mole percent, the remainder of the material being zinc oxide. This powder is ground and pressed into the desired shape after which the material is sintered at a temperature of approximately 1000.degree. C. to 1400.degree. C. After this, electrodes are applied to faces of the material. Wires are then attached to the electrode surface for connection to external circuits.
The materials and processes for making metal oxide varistors are well known in the art and are described, for example, in U.S. Pat. No. 3,962,144, issued to Matsuura et al.